The present invention relates to an apparatus for controlling the moment of firing a seismic source. Such source may be any acoustic source that is adapted to be fired on land, underwater, in the air, or even in solid rock. Air guns, gas exploders and steam guns are examples of such source.
The invention relates, in particular, to an apparatus for controlling the moment of firing a seismic source adapted to create a seismic pulse upon actuation by means forming part of an electric system comprising an electric source, a two-conductor electric cable connecting the electric source with the actuating means, electric switch means for controlling the electric connection between the electric source and the cable, and means for operating the switch means a predetermined period after a command signal has been supplied thereto.
In operating seismic sources of the above type, time delays are found between the operation of the switch means and the moment of appearance of the seismic pulse, which delays vary in length for seismic sources of different design, and even for seismic sources of the same design. Further, such delays are found to vary over the time for each individual seismic source.
It will be appreciated that delays of non-constant value cannot be tolerated when operating a plurality of seismic sources simultaneously as is often done for increasing the energy and/or the quality of the seismic pulse needed in a seismic operation, or when applying wave-generating techniques which require the firing of a limited number of sources at a very small, fixed time delay (in the order of 1-100 milliseconds) with respect to the firing of the other sources.
It will be appreciated that since each individual seismic source shows in operation a specific delay period extending between the moment of the seismic pulse generated thereby and the moment that the switch means that operates the actuating means is closed, adjustable time delay means are provided to control the operations of the switches of the various sources in a manner such that the pulses will occur at exactly the same moment or (if such would be required) with adjustable, predetermined time intervals therebetween.
To measure the specific delay period of each source in underwater operations, use has been made of hydrophones to pick up signals of the individual seismic pulses, which signals are fed by separate cables to the vessel from which the firing signal is given. Each hydrophone is arranged close to a seismic source, and by separately firing the sources, the specific delay period of each source can be determined from time to time. It will, however, be appreciated that separately firing of the sources is a time-consuming operation. Also, the number of electric conductors in the cable connecting the sources to the shooting vessel is doubled (since each source as well as the corresponding hydrophone needs a separate pair of conductors) which requires the use of a cable that is expensive, and moreover, difficult to handle in view of the diameter thereof.